THE CHEMISTRY AND PHYSICS OF THE CELL 23 



position). It makes no difference what the rest of the radicals 

 are, whether they are simple chains (leucin), or members of the 

 cyclic or aromatic series (tyrosin), or sulphur-containing bodies 

 (cystin), without exception this relation of a NH 2 group to an 

 acid radical is constant, as in this formula : 



NH 2 

 E CH COOH. 



Through this arrangement every one of the constituents of 

 the proteid molecule is provided with a group with a strong 

 basic character and a group with a strong acid character, and 

 hence it is possible for them to unite with one another in indefi- 

 nite numbers, and, because of the great variety of individuals, 

 in practically an infinite number of combinations. It is believed 

 that it is in just this way that the proteid molecule is built up. 

 By artificially uniting various cleavage products Emil Fischer 

 has succeeded in producing large molecules made up of several 

 amino-acid radicals (called by him " polypeptids ") l which 

 show some of the characteristics of the peptones, and this is the 

 nearest that investigators have yet come to synthesizing a proteid 

 molecule. The union is accomplished by the splitting off of 

 water, corresponding to the addition of water that occurs when the 

 proteid molecule undergoes cleavage. It may be illustrated by 

 showing the formation of the simplest polypeptid, glycylglycin. 



/NH 2 ^0 X NH 2 ^O 



CH 2 c - Jon + H]HN CH 2 COOH = cn 2 c HN cn 2 -- COOH + H 2 o. 



(glycocoll) (glycocoll) (glycylglycin) 



For these reasons it is believed that the proteid molecule 

 consists of great numbers of amino-acid groups, combined with 

 one another through their basic and acid radicals, and that the 

 various proteids are different from one another because they 

 contain different numbers or varieties of amino-acids. For 

 example, the globin of hemoglobin yields no glycocoll on hy- 

 drolysis, while gelatin yields 16.5 per cent. On the other hand, 

 gelatin is free from tyrosin. Some of the protamins (proteids 

 obtained chiefly from spermatozoa) yield as high as 58 to 84 

 per cent, of arginin, while the simpler amino-acids with but 

 one N (mono-amino-acids) are scanty, and most varieties are 

 lacking. 



It will be noticed that when two amino-acids unite, as seen 

 in the formation of glycylglycin, an acid radical and a basic 

 radical are still left free. In this may be seen the explanation 



1 Reviewed by Fischer, in Ber. deut. Chem. Gesell., 1906 (39), 530. 



